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Posted: Mar 04, 2008
Nanotechnology packaging improves lifetime of organic solar cells
(Nanowerk Spotlight) Nanotechnology is already part of the process through which silicon-based photovoltaic solar cells - which make up some 95 percent of the solar cell market today - produce electricity. Nanotechnology is also the focus of research and development of a new generation of solar power technology that includes organic and inorganic solar cells derived from nanocrystals that can convert sunlight into electricity at a fraction of the cost of silicon solar cells.
Silicon-based solar cells are made from a refined, highly purified silicon crystal, similar to those used in the manufacture of integrated circuits and computer chips. The high cost of these silicon solar cells and their complex production process has generated interest in developing alternative photovoltaic technologies. Compared to silicon-based devices, polymer solar cells are lightweight (which is important for small autonomous sensors), disposable, inexpensive to fabricate, flexible, designable on the molecular level, and have little potential for negative environmental impact. These solar nanocells are so small and pliable that they can be painted onto physical structures so that the windows and walls of a building may one day soon be able to generate electricity.
The big question today is to what degree polymer solar cells will be able to commercially compete with silicon solar cells. There are two major issues that need to be solved: 1) The present efficiency of organic solar cells lies at only around 6 percent, compared to up to 30 percent for the most efficient silicon cells. 2) Polymer solar cells suffer from huge degradation effects: the efficiency is decreased over time due to environmental effects such as water, oxygen or UV rays.
With the exciting vision of organic solar cells becoming a low-cost electricity source available in any size and shape, as flexible thin films and even coatings, researchers all over the world are working on making organic solar cells commercially attractive. One recent example is a novel polymer nanotube composite for packaging applications that has the potential to significantly improve the life time of organic photovoltaic devices.
"Until now, an important failure of organic solar cell technology is that such solar cells do not last very long" Dr. David L. Carroll tells Nanowerk. "Many researchers use glass encapsulants to protect plastic solar cells from the ravages of UV rays and oxygen, but this defeats the purpose of using plastic to start with. With our recent work, we have developed an all plastic coating, or barrier layer, that not only protects against oxygen and water – as most such barrier layers do – but also is highly efficient at filtering out the most damaging UV light. The result is applicable to all plastic solar technologies and will allow us to build truly flexible and mobile power systems."
Carroll is an Associate Professor at Wake Forest University in Winston-Salem, North Carolina, and Director of the university's Center for Nanotechnology and Molecular Materials. His team, together with collaborators from the Indian Institute of Technology in Kharagpur, have developed a novel Saran (a co-polymer of vinylidene chloride and acrylonitrile) based polymer nanotube composite, which shows high transparency in the visible region, good barrier properties and thermal stability, for use as an encapsulant for organic photovoltaic devices. They used boron nitride nanotubes throughout a water and oxygen barrier matrix to further add UV resistance to the barrier layer.
Ultraviolet radiation, that part of the solar spectrum that we need to protect our skin from by applying sunscreen lotion, also has a damaging effect on organic solar cells: they degrade severely under UV irradiation. It is therefore necessary to protect plastic solar cells from UV radiation, same as we do with our skin, by applying a barrier layer that filters out the damaging radiation.
"Some of the key requirements for an encapsulant for organic photovoltaic devices are transparency in the visible range, barrier to oxygen and moisture ingression, and mechanical and thermal stability" says Carroll. "Hence, the choice of the materials is very crucial. The properties required for food packaging materials have close similarity to those for encapsulants for organic solar cells. Hence, we chose Saran, a commercial food packaging polymer, as the matrix for the composite encapsulant."
For making their composite encapsulant, Carroll's team used commercial F-310 grade Saran as polymer and 0.5 to 1.5 wt% (with respect to Saran) boron nitride nanotubes as a filler material. The nanotubes possess attractive thermal properties like very high thermal conductivity and high thermal stability besides a wide band gap of 5.5 eV. Protective films were then made by either spin coating or drop casting the composites onto glass or silicon substrates.
"In testing our composite material we found that it exhibits outstanding wearability even in the harshest climates with direct sun or potentially, orbiting systems" says Carroll.
The researchers are currently subjecting their polymer nanotube composite to further real-life testing on outdoor systems. Ultimately they hope to develop a low-cost commercial dip-coat process to make this new barrier film low-cost and easily applicable.